/**
 * A state interface
 * �û������Ϲ����״̬�仯:
 * ѡ����Ʒ --> ���ɶ��� --> ����ȡ��
 */
public interface IShopState {
    public void shop();

    public void generateBill();

    public void pay();
    //protected void changeState(ShopContext c, IShopState);
}